1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or a copier, an image forming system equipped with such an image forming apparatus and a plurality of sheet processing apparatuses, and a control method for such a system.
2. Description of the Related Art
In recent years, there has been practically used a sheet processing apparatus that divides a plurality of image formed sheets ejected from an image forming apparatus (a printer or a copier) into groups, sorts and stacks them on a stacking tray so that positions of the sheets (or sheaves of sheets) of the respective groups are displaced in a width direction. In this case, the width direction refers to the direction perpendicular to the transportation direction on the in-plane of the sheet.
A conventional sheet processing apparatus with the aforementioned sorting function is provided with a processing tray at the front stage of the stacking tray, moves the sheet in the width direction on the processing tray and then moves the sheet to an elevating stacking tray to stack it thereon. A place where the processing tray is arranged, however, is limited to a position at the very front of the stacking tray, so that if the transportation path for the sheet is plural branched, the processing tray needs to be arranged on every branch.
For that reason, there has been proposed a mechanism in which a pair of rollers arranged on the transportation path for the sheet is moved in the width direction with the sheet sandwiched therebetween to displace the transportation position in the width direction of the sheet (refer to U.S. Pat. No. 4,635,920, for example). The sheet processing apparatus disclosed in the patent publication is incorporated into the image forming apparatus and a pair of ejecting rollers is moved in the axial direction to eject the sheet onto the stacking tray, ejecting and stacking the sheet. The sheet ejected onto the stacking tray through an image forming process and a fixing process is moved in the axial direction at two stages with the sheet sandwiched between the pair of ejecting rollers, thereby the sheet is sorted and stacked with a stack position displaced in the width direction on the stacking tray.
In addition, there has been proposed a sheet processing apparatus adapted to achieve high productivity (refer to a publication of the US Patent Application No. 2007/0075482, for example). The sheet processing apparatus disclosed in the patent publication transports the sheet while shifting the sheet in the width direction to a predetermined position by means of a shift transport mechanism located on the upstream side of a sheet stacking unit in the direction in which the sheet is transported. The sheet processing apparatus stacks the sheet on the processing tray with shift and previously moves a pair of aligning boards that aligns the sheet in the width direction on the processing tray to a position in response to the predetermined position of the sheet.
When an abnormal condition (error) occurs at the shift transport mechanism, the sheet processing apparatus disclosed in the publication of the US Patent Application No. 2007/0075482 nullifies the shift function of the shift transport mechanism and increases the distance between the pair of aligning boards. The increase of the distance between the aligning boards precludes the sheet from colliding with the pair of aligning boards even if the sheet is transported to the aligning boards with the sheet misaligned in the width direction.
However, since the increase of the distance between the aligning boards takes a longer time to return to a standby position after the aligning operation, a sheet feeding interval increases. This reduces the number of sheets to be processed in unit time.
In a print-on-demand (POD) system, a plurality of sheet processing apparatuses with shift transportation functions may be connected.
As described above, if an abnormal condition occurs at the shift transport mechanism in the sheet processing apparatus, the use of many functions is uniformly restricted, which lowers the ability of the system.
Further, even if the plurality of sheet processing apparatuses with shift transportation functions are connected, there is not used the shift transportation function of the sheet processing apparatus that is not designated. Accordingly, it is desirable to effectively use the shift transportation function.